I have a 36 G planted community tank with diamond tetras, black skirt tetras (+6 babies) and Harleqium rasboras plus some hitchhiker pond snails. Yesterday I noticed this on the front glass and today it has moved!

Thanks for your replies..it was just in a new place and getting smaller, but it's now gone, I think the snails cleaned it off...my tank has been cycled since January and I haven't added anything new for about a month now, just some new plants that I got from a friend with a planted tank. I'm going to keep a close watch and see if it shows up again.